This invention relates to mobile communication and as an example refers to the currently discussed and developed Long-Term Evolution standard (LTE) which features, among others, coordinated transmissions from multiple points also known as Coordinated Multipoint Transmission CoMP.
While previously CoMP required coordination between cells using the same carrier, in most recent CoMP scenarios, the transmission points to be coordinated can be of different locations or different antennas (e.g. in MIMO arrangements (Multiple Input Multiple Output). Likewise, such scenarios may involve that the locations are assigned the same cell ID or resort to resource using different component carriers CC (like e.g. in carrier aggregation). CoMP as used in the present invention is thus to be understood in its broadest sense and not limited to pre-existing ComP scenarios.
Preceding standardization discussions selected the HetNet (heterogenous network) scenario as one of the basic scenario for CoMP evaluation. In this scenario, several Pico cells (constituted by pico eNB (evolved Node_B) or, simply, remote radio heads RRH) locate in the coverage of a macro eNB. Pico and macro, respectively, denote the coverage area defined by such a transmission point or access point, and the coverage area is linked to the transmit power assigned to such nodes which is also reflected in the naming adopted, i.e. pico cells have smaller coverage while macro cells have larger coverage.
The cooperation or coordinated transmission from multiple such points, i.e. CoMP, could either happen between Macro eNB and one or more Pico eNBs in corresponding coverage, or between Macro eNB/Pico(s) of different sectors/cells. (Note that pico eNB sometimes is simply referred to as remote radio head RRH.)
For explanatory purposes, FIG. 1 illustrates roughly a heterogeneous network with low power (small coverage) RRHs within the macrocell coverage. In this exemplary scenario, transmission/reception points created by the RRHs have different cell IDs as the macro cell. The coordination area includes 1 cell with N low-power nodes as starting point, i.e. as illustrated 3 intra-site cells with 3=N low-power nodes. The macro eNB is assigned cell ID 1, and the RRH are assigned their respective individual ID, i.e ID 2, ID 3, and ID 4. The benchmark for the expected performance is non-CoMP Rel. 10 eICIC framework (enhanced intercell interference coordination with the different cell ID.
Likewise, for explanatory purposes, FIG. 2 illustrates roughly a network with low power RRHs within the macrocell coverage, where the transmission/reception points created by the RRHs have the same cell IDs as the macro cell. Here, the coordination area includes 1 cell with N low-power nodes as starting point, and thus 3 intra-site cells with 3=N low-power nodes. Also here, the performance benchmark is non-CoMP Rel. 10 eICIC framework with the different cell ID.
The major difference of these scenarios shown in FIG. 1 and FIG. 2 is whether different transmission points have the same cell-ID or different cell-ID.
The basic deployment scenario is one macro eNB+multiple RRH with fiber connected. (The fiber connections are illustrated in FIG. 2 between macro and pica points as arrows, but are omitted from illustration in FIG. 1). There are two basic methods to assign cell-ID for all the transmission points (1 macro+N RRH in one cell area).
In FIG. 1, transmission points are of different cell ID: Different sync, broadcast and UE-specific control channels are thus transmitted from each point, each RRH is an individual cell seen from UE.
One advantage of having different cell ID's for different physical cells (or physical transmission points) is a maximized PDCCH multiplexing capability (physical downlink control channel). Without cell-specific scrambling sequence and the interleave feature of the PDCCH, different cells (different transmission points) can multiplex the same resource for different PDCCH transmission.
However, the most challenging issue of different cell ID for Rel. 10 and later releases, is DMRS (demodulation reference signal) orthogonality issue. If a different cell ID is used, the DMRS used for PDSCH (physical downlink shared channel) is always quasi-orthogonal between each other. That is not an optimized case comparing with same-cell ID approach. E.g. In same cell ID case, a scheduler can assign the same physical resources to multiple UE and each with a mutually full-orthogonal DMRS sequence. In other words, the different DMRS sequence based on the same-cell ID can provide spatial orthogonality between different UEs.
That's also the reason at Rel.8, it's per UE DMRS, and in Rel.9, it's per cell DMRS with dynamical indicated scrambling ID.
A second option is shown in FIG. 2 as proposed in a recent 3GPP RAN1 meeting. All the transmission points (macro and RRH) within the coverage area of a macro point share the same cell-id, (one cell). It is important to realize that adding pico points to an existing macro deployment does not increase the area covered by a cell.
The major advantage of this same cell-ID concept is to extremely enlarge the degree of freedom for data channel cooperation transmission, either from CSI-RS (channel state indicator reference signal) or DM-RS (demodulation reference signal) perspective. Because the same cell-id provide the same base sequence for CSI-RS and DM-RS, and different port of CSI-RS/DM-RS provide perfect orthogonality.
But the disadvantage is also obvious, namely that all the UEs in the coverage of Macro eNB including those in the Pico coverage should share the same PDCCH resources, which will extremely limit the degree of freedom and also limit the potential gain of cooperation transmission.
Either options (same cell-ID or different cell-ID) has its own pros and cons, so it's really hard to make final selection because it's hard to evaluate which is more influential for the compromise that has to be made.
The fundamental reason for above situation is that no matter same cell-ID or different cell-ID solution, PDCCH/PSS (primary synchronization signal)/SSS (secondary synchronization signal)/BCH (broadcast Channel)/PCFICH (Physical control format indicator channel)/PHICH (physical hybrid ARQ indicator channel) and CSI-RS/DM-RS/PDSCH from the same transmission points are always connected to the same cell-ID. So if control channels (PDCCH, PHICH, and all CRS (common reference signal) based channels) are deployed with same cell ID, then all the CSI-RS and DMRS will be of same cell ID as well. That makes control channels are limited by multiplexing capacity since all the sequences for control channels are based on the same sequence. If control channels (PDCCH, PHICH, and all CRS based channels) are deployed with different cell ID, then all the CSI-RS and DMRS will be of different cell ID as well. That makes the DMRS/PDSCH from different transmission impossible to be orthogonal to each others, which will limit the potential gain of cooperation transmission again.
Some companies have proposed using some cell specific CoMP ID to address DL (downlink) DMRS sequence ID indication. That is to say, instead R10 cell ID, using (an additionally provided) CoMP ID to generate the sequence ID for the UE in the CoMP transmission.DMRS_ID=f(CELL_ID+Scramble_ID)  R10 UEDMRS_ID=f(CoMP_ID+Scramble_ID)  CoMP UE
This method has addressed the problem above but partially. Namely, according to this method, DMRS sequence ID can still not be changed dynamically, (and the scrambling ID is quite limited). Considering a large complex CoMP coordination area, flexibility on cell ID used is quite needed to achieve best performance.
Therefore, it is an object of the present invention to propose improvements in scenarios as outlined above.